This invention relates generally to the separation of gases, and more particularly to the separation of CO2 from the exhaust gases of fossil-fueled power plants.
Carbon dioxide emissions have been identified as a major contributor to the phenomenon of global warming. The removal of this so-called greenhouse gas from the exhaust stream of fossil-fueled power plants is a major ecological and economic issue. There exists to date no method or device for removing CO2 from the exhaust stream of fossil-fueled power plants which satisfies the needs of efficiency and economy. Gas separation technology is an old and well-developed technology, however, prior gas separation technologies cannot separate CO2 from the emissions of fossil-fueled power plants economically.
Natural gas is the cleanest burning of fossil fuels with respect to emission of acid gases such as sulphur dioxide and carbon dioxide. For example, compared to coal, the burning of natural gas results in the emission of only 60-70% of the CO2 emissions of a coal burning system. For the past several years, the perceived abundance of natural gas, advances in gas turbine technology, and many other factors have resulted in significant increases in the use of natural gas for power generation. However, considerable quantities of sub-quality natural gas exist in the United States, and this must be upgraded prior to use. Carbon dioxide is an impurity that creates operational, economic, and environmental problems. It is a diluent without any fuel value, and is an environmental concern as it is one of the greenhouse gases. It is an acid gas and can cause corrosion problems in the presence of water, creating carbonic acid that is quite corrosive to some alloys.
Several CO2 separation and capture technologies have potential for the purification of natural gas. These include amine scrubbing, molecular sieves, cryogenic removal, and membrane separation. Molecular sieves, such as zeolites and activated carbon, are used in pressure swing adsorption (PSA) or temperature swing adsorption systems which separate gas mixtures by selective adsorption of one or more of the gases at high pressure and/or low temperature thus producing a pure product stream. The captured gas is then desorbed by lowering the pressure, or increasing the temperature, of the adsorbent system (thus the system xe2x80x9cswingsxe2x80x9d from a high to low pressure or a low to high temperature). The desorption step regenerates the adsorbent material for reuse during the subsequent adsorption step.
PSA systems typically comprise several adsorption beds, through which the gas stream is passed, allowing for the near complete separation of the selected gas species. The adsorbent materials used in a PSA unit are selected to have the appropriate mean micropore width (typically in the range of 5-10 xc3x85) to selectively adsorb or sieve the required gas species and additionally must possess large surface areas. Currently available adsorbent materials include zeolites with surface areas in the range of 10-350 m2/g, and activated carbons with surface areas in the range of 500-1000 m2/g.
High service-cycle costs have limited the implementation of many technologies for air quality improvements as in the case of activated carbon systems. The effective life of each sorbent depends on both the amount of pollutant captured and the sorptive capacity of that material. Major technical and operating problems associated with granular sorbents include channeling, settling (packing), and resistance to air flow. Conventional activated carbons and carbon molecular sieves are granular in structure. During operation in a PSA system, granular materials suffer attrition and can settle resulting in the formation of channels which allow the fluid stream to bypass the adsorbent. Lower life cycle and service cycle costs are needed to meet the demands of rapidly growing residential and commercial markets.
A new material for filtering gas streams to separate gaseous components of the stream is known as a carbon fiber composite molecular sieve (CFCMS). CFCMS air filter media is an activated carbon media which is described in U.S. Pat. Nos. 5,827,355 and 6,030,698, the disclosures of which are incorporated herein by reference. This patent describes a CFCMS material with a density in the range of about 0.3-0.4 g/cc. This composite is activated to produce a significant volume of mesopores (2-50 nm) and/or micropores ( less than 2 nm). The rigid structure has macropores in the range of 10-500 microns which allow for excellent fluid flow through the sample, resulting in an acceptable pressure drop. The rigid nature of the composite also eliminates problems due to channeling and settling. The material has a continuous carbon structure and is electrically conductive. The passage of electric current, typically 1-20 amps at 1-5 volts for a small segment of media, causes the carbon fiber composite molecular sieve to heat, thus electrically and thermally desorbing sorbed gases.
Cartridge filters have been used in various applications for banked filtration of process gas streams, such as gas turbine exhaust systems and dust collection systems. The cartridges can have several different designs. In one design, the cartridges are essentially tubular and the gas stream flows into contact with the outer surface of the filter and the clean air flows out through the center of the cartridge. The cartridge is periodically purged with a discharge stream of pressurized gas, or by the PSA method. A cartridge filtration system is shown in U.S. Pat. No. 5,961,696, the disclosure of which is hereby incorporated by reference.
A device for separating gases, and particularly for removing CO2 from fossil-fueled power plant emissions, includes concentric inner and outer conduits defining an inner passage and an annular outer passage. A filter media fills at least a portion of the annular outer passage. The filter media is electrically conductive and preferentially adsorbs at least one of the constituents of the gas stream. Gas flows sequentially through the inner passage and then the outer passage, or through the outer passage and then the inner passage. In the outer passage, the gas contacts the filter media such that the desired gas, such as CO2, is preferentially adsorbed. The filter media is regenerated by applying a power supply to a circuit connecting the conductive filter media, the inner conduit and the outer conduit. The inner and outer conduits are electrically insulated from one another, and the circuit connects the inner and outer conduits through the conductive filter media. Current flows through the conductive filter media to physically desorb the gas from the filter media. A purge gas or vacuum can be applied to facilitate removal of the gas from the filter media. A preferred filter media is CFCMS.
A method for removing CO2 from fossil-fueled power plant emissions includes the steps of flowing the emissions through an inner conduit and through an outer conduit that is concentric to the inner conduit. The gas contacts a filter media in the annular space between the inner and outer conduits. The filter media, preferably CFCMS, is electrically conductive and preferentially adsorbs a desired constituent of the gas stream such as CO2. The CO2 is adsorbed onto the CFCMS, and the product gas stream is vented or sequestered. The CFCMS is regenerated by causing an electric current to flow through the CFCMS. The inner and outer conduits are preferably electrically insulated and conducting, and an electric current is caused to flow between the inner and outer conduits and through the CFCMS to heat the CFCMS and desorb the CO2 from the CFCMS so as to regenerate the CFCMS. A purge gas and/or vacuum can be applied to facilitate the desorption of the CO2 from the CFCMS. The desorbed gases, such as CO2, are vented or sequestered.